With an increasing complexity of light emitting diodes (LEDs), and in particular, ultraviolet light emitting diodes (UV LEDs) based on group III nitride semiconductor layers, aspects of reliability and lifetime, and thus the costs associated with light emitting diodes, become increasingly important. Reliability and lifetime can be a basic decision factor for selection of a source for UV LEDs. Reliability is a significant concern that presents a challenge during optimization of epi structures and device package designs (such as heat management). The lifetime and reliability of UV LEDs are dependent on many factors, such as design flaws of a UV LED system component, manufacturing defects, wear-out mechanisms, and/or the structure of epi designs for the UV LEDs. The failure of any LED system component—not just including the array of LED packages, but also the electronics, thermal management, optics, wires, connectors, seals, or other weatherproofing, for example—can directly or indirectly lead to product failure. Further, while some LED products will fail in a familiar catastrophic way, others may exhibit parametric failure, in which the LED product stops producing an acceptable quantity or quality of light.
Fabrication of a high-quality aluminum gallium nitride (AlGaN) epitaxy continues to be challenged by a lack of matched substrates. Threading dislocations that result from heteroepitaxy are responsible for leakage currents, trapping effects, and may adversely affect device reliability. AlN nucleation conditions can be important for reliability of the device when grown on silicon carbide (SiC) substrates. For example, variation of the nucleation temperature, V/III ratio, and thickness can have a dramatic effect on the balance between edge, screw and mixed character dislocation densities. Electrical and structural properties have been assessed by AFM and XRD on a material level and through DC and RF performance at the device level. The ratio between dislocation characteristics has been established primarily through comparison of symmetric and asymmetric XRD rocking curve widths.
One prior art study evaluated microstructural evolution, with particular emphasis on threading dislocation generation, in a two-step metal-organic chemical vapor deposition (MOCVD) of GaN on sapphire. MOCVD growths were carried out at atmospheric pressure in a horizontal two-flow reactor. Nominally, 200 Angstrom (Å) thick nucleation layers were deposited at temperatures in a range of 525-600° C. followed by high temperature growth at temperatures in a range of 1060-1080° C. Throughout the different stages of growth, the microstructure was studied by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Two growth conditions were closely studied: brief pre-growth ammonia exposure of the sapphire (‘Material A’) and extensive pre-growth ammonia exposure of the sapphire (‘Material B’).
The study concluded that nucleation layer growth conditions and the resulting microstructure, after high temperature exposure, critically impact the GaN microstructure. The work showed that if the nucleation layer after high temperature exposure has threading dislocations, then these threading dislocations will propagate into the high temperature GaN and it is likely that the resulting GaN will have a high threading dislocation density (e.g., 1010-1011 cm−2). Material A results demonstrate that the optimal nucleation layers to achieve low threading dislocation densities (e.g., 108-109 cm−2) have rough morphologies and a high-degree-of-stacking disorder after high temperature exposure. Nucleation layers with these characteristics then provide a template for aligned growth of nearly perfect high temperature GaN islands. The high temperature islands appear to grow by a spiral mechanism. The majority of threading dislocations are subsequently generated at the coalescence of the high temperature islands.
While this work is relevant to the growth of AlGaN-based ultraviolet light emitting diodes, the growth conditions for AlGaN devices are significantly different from the growth conditions of devices rich in gallium.
A significant obstacle to improving the performance of DUV LEDs is the high threading dislocation density present in the LED structures due to heteroepitaxial growth on foreign substrates. Moreover, AlN and AlGaN films grown on such substrates beyond critical thicknesses suffer from cracking due to a strong tensile strain. While native AlN substrates alleviate this issue, presently commercially available AlN substrates are cost prohibitive, have usable area limitations, and suffer from excessive absorption. Meanwhile, alternate approaches, such as epitaxial lateral overgrowth (ELOG) of AlN and AlGaN, are being used to deposit thick films and limit the threading dislocation density in epilayers. The ELOG technique has been successfully employed to deposit several hundred micrometers thick crack-free GaN films, which demonstrate a 2-3 orders of magnitude reduction in the threading dislocation density. However, the lateral overgrowth often causes problems related to the coalescence of growth fronts, such as dislocation generation at the coalescence points due to wing tilts and stress relaxation. Devices produced on films with coalescence-related defects exhibit poor yield, making the approach impractical for commercial application.
Therefore, the coalescence of the growth fronts must be carefully controlled to prevent defect generation in large area films sufficient for device fabrication. To improve the quality of the semiconductor layers grown on sapphire substrates, a previous approach proposed growth of low-defect thick films of AlN and AlGaN on trenched AlGaN/sapphire templates using migration enhanced lateral epitaxial overgrowth. Incoherent coalescence-related defects were alleviated by controlling the tilt angle of growth fronts and by allowing aluminum adatoms sufficient residence time to incorporate at the most energetically favorable lattice sites. Deep ultraviolet light emitting diode structures (310 nm) deposited over fully coalesced thick AlN films exhibited a cw output power of 1.6 mW at 50 mA current with extrapolated lifetime in excess of 5000 hours. The results demonstrate substantial improvement in the device lifetime, primarily due to the reduced density of growth defects.
A study evaluated characteristics of AlGaN-based deep ultraviolet light emitting diodes (DUV LEDs) grown on a high-quality AlN/sapphire template (AlN template). LED structures were grown directly on a two inch diameter AlN template by metal-organic chemical vapor deposition. AlGaN epilayers were confirmed to have a high crystal quality on the AlN template through X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM). The fabricated LEDs exhibit a sharp single peak emission at a DUV region around 260-270 nm from electroluminescence spectra measured at room temperature. The light intensity and current-voltage characteristics are improved by using higher quality AlN template as the underlying substrate. This approach could facilitate the production of high-performance DUV LEDs.